Paper has recently been put to numerous applications in an extensive range, and paper stock having functions never heard before has been developed up to date.
As a result, paper produced by a paper machine has come to be diversified, and is numerous in type. As paper produced by a paper machine has to go through a drying process before turned into a product, the drying process occupies a very important position from the technical viewpoint.
The paper machine is equipped with a plurality of dryers used for drying, and the dryers have a construction such that a heating medium such as steam and the like are contained therein for heating from inside of the dryers.
Moist paper undried as yet is fed continuously to the dryers of the paper machine.
It is a canvas as it is called that plays a role of pressing hard wet paper into contact with the surface of the dryers.
The wet paper gives off moisture contained therein by absorbing heat from the surface of the dryers when pressed by the canvas.
For this reason, the canvas is normally formed of material flexible and porous, for example, woven fabric, felt (unwoven fabric), knitted fabric, and the like.
An enlarged view of the surface of the canvas shows that a multitude of fine pores (so-called eyes of the canvas) are formed on the surface thereof between threads making up the canvas.
Moisture contained in the wet paper is heated by the dryers at high temperatures, and dispersed through the eyes of the canvas.
Incidentally, paper contains various substances, for example, pitch/tar component contained in pulp stock itself, hotmelt, ink and microfibers originating from waste paper stock, various additive chemicals for reinforcing strength and whiteness of paper, and inclusions (foreign matter) such as paint, and so forth. Such inclusions having tackiness will be deposited on the surface of the canvas when the wet paper is pressed hard against the surface of the dryers by the canvas, turning into contaminant (the source of contamination).
That is, the inclusions contained in paper undergo denaturization due to the effect of pressure and heat, and adhere to the surface of the canvas, causing the eyes of the canvas to be clogged.
There has since been a shift in raw material for the canvas from natural fiber to synthetic fiber with the result that the canvas has come to have a longer service life due to longer durability thereof.
The longer service life means that there will be an increase in accumulation of the contaminant to that extent.
Accordingly, after the operation of the paper machine for a long duration, there will occur fairly often a phenomenon wherein the eyes of the canvas get clogged with the contaminant.
In recent times, addition of, for example, polyacrylic resin to paper has been in practice curing a paper making process in order to reinforce paper strength and to improve a product yield, and particularly, in the case of an additive of a cation type being used, the additive is easily transferred to the canvas, and adheres thereto, causing the eyes of the canvas to be clogged.
Once the clogging of the eyes of the canvas occurs, air permeability of the canvas deteriorates drastically, resulting in poor drying efficiency of paper.
More specifically, sufficient moisture is normally emitted from the wet paper through the eyes of the canvas when the wet paper is pressed hard between the dryers and the canvas, however, once the clogging of the eyes of the canvas occurs, emitted moisture can not find its way out. In such a state, the dig efficiency deteriorates significantly.
Parts of the contaminant accumulated on the canvas are then transferred from the surface of the canvas to a portion of a paper strip, newly fed, resulting in contamination of product paper.
The contaminant having strong tackiness is also accumulated on an out-roll serving as a guide-roll for the canvas and turned into large lumps.
When parts of such lumps are peeled off, and adhere to the surface of the wet paper, the wet paper will be provided with additional tensile force due to the effect of the tackiness of the contaminant at the time of the dryers coming in contact with the wet paper, creating a cause for breaks of the wet paper.
For solving such problems as described above, countermeasures have been adopted wherein periodical cleanups of the canvas are carried out frequently or an interval between replacements of the canvas is rendered shorter. These countermeasures, however, have turned out to be expensive in terms of time and cost.
Accordingly, there has been developed a method of preventing contamination of the canvas by applying surface treatment to the canvas.
That is, it is a method whereby the surface of the canvas is treated with a water-repellent or of repellent resin, for example, ethylene resin tetrafluoride.
Such treatment, however, will result in formation of regions where the eyes of the canvas are partially clogged, causing the drying efficiency to be deteriorated.
Furthermore, such an anti-fouling treatment will be effective only in the initial stage of operation of the paper machine, and the effect of the treatment will decrease in several days (for example, in 6 to 6 days after the treatment is applied), so that a treated canvas then will not be different at all from the canvas untreated.
As described in the foregoing, the paper machine has been faced with major technical problems in that various inclusions contained in the wet paper are adhered to the canvas, and accumulated thereon along with the operation of the paper machine, whereupon the eyes of the canvas are clogged, thereby biting evaporation of moisture, and resulting in poor drying efficiency, and also in that the contaminant accumulated on the canvas as well as the out-roll causes parts of the paper strip, newly fed, to be contaminated as well or create a cause for breaks of the paper strip being processed.